Underfill formulations are widely used in the electronics industry to bond various components, such as flip chips, to substrates. Typically, an underfill formulation is handled and applied at room temperature, or heated to an appropriate working temperature, where the formulation flows under the chip by capillary action. It is desirable therefore that underfill materials have good flow properties prior to curing, as well as good performance properties during and after curing.
Another common use of resins in the electronics industry is as a liquid encapsulant (also referred to as “glob top”), wherein an aliquot of resin material is used to encase (or encapsulate) a component to protect it from certain stresses and from exposure to the environment. To meet the industry's ever-increasing demand for device reliability, materials for encapsulant applications must meet increasingly stringent performance requirements. Such requirements include excellent moisture resistance, ionic purity, low dielectric constant and good thermal properties. In the absence of these properties, especially in the presence of moisture and ionic impurities, corrosion (and ultimately failure of the device) will likely occur at some point during the life of the device.
Underfill and encapsulant formulations are generally quite similar in composition, differing primarily in their end use. Whereas underfill formulations are employed to protect the solder bumps under a chip (and to provide a material of intermediate coefficient of thermal expansion between the chip and the substrate, thereby reducing stress), encapsulant formulations are employed to protect exposed components (e.g., wire bonds and components on the top of a chip), which components would otherwise be exposed to environmental factors such as heat, moisture, particulate matter, and the like.
In any event, both underfill formulations and encapsulant formulations oftentimes suffer from reduced reactivity in the presence of flux or flux residues. “Flux” refers to agents which promote the fusion of metals, and thus are commonly encountered in processes where electronic components are being fabricated. Flux residues refer to derivatives, decomposition products, and the like, of fluxing agents, as a result of such processes as hydrolysis, thermolysis, and the like. Flux or flux residues are undesirable because they are capable of chemically reacting with underfill formulations, potentially changing the characteristics thereof, e.g., reducing the adhesion properties, degrading the mechanical, thermal and/or chemical resistance thereof, and the like. Flux or flux residues can also cause poor flow properties, making handling such as dispensing of the formulation, difficult. In addition, flux or flux residues can lead to a propensity of such formulations to form voids upon cure, which may produce weakness in the resulting bond and/or a gap in the protection afforded by encapsulation.
Accordingly, there remains a need for formulations which display improved performance properties, such as good flux compatibility, improved flow properties, improved voiding characteristics, and the like.